The present invention relates to a method of and apparatus for applying a liquid to a roller. More particularly this invention concerns an inking or wetting mechanism for a printing roller.
In many printing processes a liquid, either ink in a normal printing process or water in an offset process, must be applied in an even coat to a roller, hereinafter referred to as the applicator roller. The liquid is usually held in a bath in which is partially immersed a pickup or ductor roller. A third roller, referred to as the fountain and transfer roller, is in constant radial contact with the applicator roller and is oscillated about the axis of the applicator roller so as to be brought into periodic contact with the pickup roller from which it takes liquid. The applicator roller usually rotates in the opposite sense as the pickup roller and drives the transfer roller so that it rotates in the same rotational sense as the takeup roller. Such mechanism is described in U.S. Pat. No. 2,788,741 or German Pat. No. 1,093,804. Since the transfer roller and the pickup roller rotate in the same rotational sense, their peripheries will move in opposite tangential directions at the point of radial contact between these two rollers.
Such arrangements have several disadvantages. First of all, they are subject to considerable wear since the drive for the transfer roller normally is only effective by friction contact with the applicator roller so that considerable load is put on the system when the transfer roller engages the pickup roller. Furthermore, the ink exchange from the pickup roller to the transfer roller invariably results in an isolated mass of ink or liquid to be applied to the transfer roller. Subsequent transfer of this ink or liquid to the applicator roller results in uneven printing results. Nonetheless, for most even transfer it is necessary that the relative peripheral velocities between the pickup roller and the transfer roller be maximized, and that the length of time they remain in contact similarly be maximized. For this reason the scraping effect between these two rollers, which leads to premature wear and breakdown of the system, must be augmented in order to receive best results. Even so, liquid is frequently applied to the transfer roller to in an uneven manner, requiring the applicator roller to in turn transfer it to several further applicator rollers in order to form a film of sufficiently uniform thickness for good printing.